The group claiming responsibility for the deadly attack on Kenya's Westgate mall, al-Shabab, is a murky, fractured organization with links to al Qaeda and a host of members born and raised in America. Here are some other key facts about the militants propelled by bloodshed to headlines around the world today:

What Is Al-Shabab?

Al-Shabab is a Somalia-based terror group that was originally formed in the mid-2000s as the militant wing of the Somalia Council of Islamic Courts, an organization that managed to take over much of the southern half of Somalia by late 2006, according to the National Counter-Terrorism Center. Security analysts said al-Shabab grew in popularity as they fought Ethiopian forces operating in Somalia that were accused by Somalis of a host of abuses.

In 2008 the U.S. government officially designated al-Shabab as a terrorist organization, alleging it was made up of a "number of individuals affiliated with al Qaeda" and some senior leaders who have trained and fought with al Qaeda in Afghanistan. Al Qaeda leader Ayman al-Zawahiri announced a formal alliance between the two groups in February 2012.

Al-Shabab has claimed responsibility for several deadly terror attacks in the region before, including the 2010 bombing in Uganda that killed dozens of people, including a 25-year-old American, who had gathered to watch the World Cup on large-screen TVs.

Accused by the U.S. of the assassinations of civilians and more indiscriminant bombings, al-Shabab ruled over a large portion of southern Somalia and much of its capital, Mogadishu, until a United Nations-backed force from the African Union was able to beat them back out of the capital and other large strategic cities in 2011 and 2012. The African Union troops included those from neighboring Kenya and Uganda.

As the group was forcibly pushed out of major urban areas in Somalia, analysts say it suffered its own political infighting about its long-term goals -- the group fractured between those who wanted to focus on taking back Somalia and those who dreamed of international jihad. A hint of that struggle was revealed in an oddly public spat between al-Shabab's leadership and one of its most high-profile American members, the rapping jihadist Omar Hammami. Hammami was reportedly killed by rivals in al-Shabab last week. Months before he had released an online video in which he said he feared for his life after arguing with the leadership "regarding matters of the Sharia [Islamic law] and matters of strategy."

Still, the State Department estimates al-Shabab has "several thousand" members "when augmented by foreign fighters and allied clan militias."

A U.S. official told ABC News today that "despite being severely weakened as an insurgency, al-Shabaab's lethality as a terrorist outfit has been fairly constant."

"The assault on the Westgate mall in Kenya was the most prominent in a string of terrorist attacks in Somalia and Kenya stretching back over the past two years," the official said.

Yes, al-Shabab recruits fighters from all over the world and is believed to have successfully lured in dozens of Americans to their cause in the past years. U.S. officials told ABC News that a recent estimate put the number of Americans fighting for the group at approximately 50.

Ali Soufan, a former FBI agent and terrorism expert, said some of the Somali-Americans are drawn back to the land of their fathers to fight what they see as wrongdoing there by African troops.

"They look at the situation in Somalia as a nationalistic cause," he said. "So they are there to fight to liberate Somalia from African occupiers."

In another case, revealed in court documents in late 2012, three former recruits who survived a trip to Somalia and returned to the U.S. testified they were talked into fighting for al-Shabaab by charismatic, devout older men who promised paradise for those who died in combat against "invaders."

Al-Shabab also leverages its American members to help recruit more of their countrymen with online videos. In addition to Hammami's rapping videos, which espouse the glory of jihad, as recently as last month al-Shabab publicized a video of three Americans who they say were later killed.

"This is the best place to be honestly," one of the men says. "I can only tell you from my experience being here, that you have the best of dreams, you eat the best of food, and you're with the best of the brothers and sisters who came here for the sake of Allah. If you guys only knew how much fun we have over here. This is the real Disneyland, you need to come here and join us and take pleasure in this fun."

Fighting Invaders? Then Why Attack a Mall Full of Shoppers?

In its claim of responsibility for this attack, a spokesperson for al-Shabab said the attack was meant to punish Kenyans for their military's intervention in Somalia in recent years.

David Shinn, former U.S. Ambassador to Ethiopia, told ABC News that while the ongoing attack shows a certain amount of determination, training and organization, he saw the target selection as a sign of weakness in the fractured group.

"I wouldn't call it a last desperation effort, but quite frankly I would describe it as an indication of weakness on their part," he said. Shinn said it shows al-Shabab "can only get in the headlines if it attacks a soft undefended civilian location in a neighboring country -- it's an indication they're not able to go toe-to-toe with the African Union forces in Mogadishu or in Somalia generally."

The U.S. official who spoke to ABC News said it is "really too early to say if al-Shabab's latest attack is the beginning of a broader campaign in Kenya or a desperate attempt to compel Nairobi to withdraw its troops from Somalia."